This invention relates to a circuit arrangement for adjusting the operating voltage of high-pressure gas discharge lamps to a given nominal value comprising a controllable current limiter arranged in series with the lamp and an adjustment circuit controlling the current limiter. The output signal of the adjustment circuit is determined by the difference between a given desired value determining the nominal operating voltage and an actual value depending upon the instantaneous lamp operating voltage.
During operation of high-pressure gas discharge lamps, problems arise due to fluctuations of the lamp operating voltage, for example due to a variation of the AC house voltage and due to a variation of the lamp operating voltage during the life of the lamp, the latter being an ageing phenomenon. This may lead to an overload of the lamp and hence to a shortening of its life. Moreover, in sodium high-pressure discharge lamps having an increased sodium pressure an undesired effect occurs in that the color properties of the lamp also vary with the lamp operating voltage. Differences in the lamp operating voltage occur not only due to the aforementioned ageing phenomena during the lamp life, but may also be caused by manufacturing tolerances. Frequently, the lamp operating voltage increases during its life. However, a decrease of the operating voltage or a combination of both effects may also occur during this time.
Therefore, an adjustment of the lamp operating voltage to a given nominal value is also to be understood to mean that for each lamp of a given type, if operated by the aforementioned circuit arrangement, the same value of the operating voltage is adjusted. This does not correspond to a power stabilization because on account of manufacturing tolerances different power consumptions of the individual lamps may be required for obtaining the same lamp operating voltage. For the same reason, it is not possible either to stabilize only the lamp current.
U.S. Pat. No. 4,475,065 discloses a circuit arrangement of the kind mentioned in the opening paragraph, in which the operating voltage of high-pressure gas discharge lamps is adjusted when their instantaneous operating voltage exceeds the desired nominal operating voltage by more than 10%. For this purpose, a complicated and expensive adjustment circuit is used. However, a readjustment is not effected in the case of a decrease of the lamp operating voltage. Per volt variation of the lamp operating voltage, in the known circuit arrangement a variation of only 1.5% at most of the lamp power is permissible for adjustment of the lamp operating voltage because otherwise instabilities occur in the lamp.
However, such a small lamp power variation is not sufficient in all lamp types in order that variations of the lamp operating voltage can be compensated for satisfactorily.